Various types and configurations of seal arrangements, bushings, packing boxes, stuffing boxes and the like are known for providing a seal around the rotating shaft that drives a ship's propeller, where the shaft penetrates through the stern of the ship from the interior to the exterior of the ship's hull. The seal arrangement must effectively prevent water from the exterior environment, i.e. from the exterior of the hull below the waterline, leaking into the interior of the hull where the shaft penetrates through the hull, while still allowing the shaft to rotate. Preferably, the seal arrangement should also dissipate the frictional heat that arises due to rubbing friction of seal elements on the rotating shaft, and should allow for the removal of wear particles or other contaminants from the seal arrangement, and further should provide a long operating life and allow easy maintenance.
One known type of seal arrangement uses plural seal rings spaced axially from one another along the propeller shaft in a stern tube, thereby forming successive seal chambers respectively between successive ones of the seal rings. An outermost chamber can be filled with seawater, an intermediate chamber can be pressurized with a compressed gas such as compressed air, and a third inner chamber can be filled with oil. Such an embodiment of a seal arrangement is known from the German Patent DE 44 34 247, for example, and also see German Patent DE 37 42 079. The entire disclosures of these two German Patents are incorporated herein by reference.
In practice, it has been found that further improvements would be desirable. For example, in large ships, the oil lines supplying oil to the oil-filled chamber must be quite long, which adds to the complexity, cost and weight of the overall system.
In such known systems, there is also the danger of air passing from the pressurized air-filled chamber into the adjacent oil-filled chamber. This can cause air bubbles and foaming in the lubricating oil, and it may not be possible to completely remove these air bubbles, foam or gas inclusions from the lubricating oil via a ventilation or de-gassing line. As a result, the oil may provide an inadequate lubricating effect, which leads to more-rapid wear and premature failure or damage of the seal rings. Furthermore it has been found that the oil present in the oil-filled chamber ages prematurely and loses its required lubricating properties due to strong thermal and mechanical loads and demands being placed on this oil. Also, this oil can become contaminated with wear particles caused by wear of the seal rings and the bushing, and also can become water-contaminated due to penetration of water past the seal rings into the oil-filled chamber. It is not possible to regularly exchange the oil in the oil-filled chamber, so that after a certain time of operation, the oil's lubrication effect of the seal rings is no longer optimal, and due to the breakdown of the oil over time a chemical degradation of the seal rings may also arise.